////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 1993-2021 The Octave Project Developers
//
// See the file COPYRIGHT.md in the top-level directory of this
// distribution or <https://octave.org/copyright/>.
//
// This file is part of Octave.
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// under the terms of the GNU General Public License as published by
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////////////////////////////////////////////////////////////////////////

#if defined (HAVE_CONFIG_H)
#  include "config.h"
#endif

#include <cassert>
#include <cmath>

#include <limits>
#include <ostream>

#include "Array.h"
#include "CollocWt.h"
#include "lo-error.h"
#include "lo-mappers.h"

// The following routines jcobi, dif, and dfopr are based on the code
// found in Villadsen, J. and M. L. Michelsen, Solution of Differential
// Equation Models by Polynomial Approximation, Prentice-Hall (1978)
// pages 418-420.
//
// Translated to C++ by jwe.

namespace octave
{
  // Compute the first three derivatives of the node polynomial.
  //
  //                 n0     (alpha,beta)           n1
  //   p  (x)  =  (x)   *  p (x)         *  (1 - x)
  //    nt                   n
  //
  // at the interpolation points.  Each of the parameters n0 and n1
  // may be given the value 0 or 1.  The total number of points
  // nt = n + n0 + n1
  //
  // The values of root must be known before a call to dif is possible.
  // They may be computed using jcobi.

  static void dif (octave_idx_type nt, double *root, double *dif1,
                   double *dif2, double *dif3)
  {
    // Evaluate derivatives of node polynomial using recursion formulas.

    for (octave_idx_type i = 0; i < nt; i++)
      {
        double x = root[i];

        dif1[i] = 1.0;
        dif2[i] = 0.0;
        dif3[i] = 0.0;

        for (octave_idx_type j = 0; j < nt; j++)
          {
            if (j != i)
              {
                double y = x - root[j];

                dif3[i] = y * dif3[i] + 3.0 * dif2[i];
                dif2[i] = y * dif2[i] + 2.0 * dif1[i];
                dif1[i] = y * dif1[i];
              }
          }
      }
  }

  // Compute the zeros of the Jacobi polynomial.
  //
  //    (alpha,beta)
  //   p  (x)
  //    n
  //
  // Use dif to compute the derivatives of the node
  // polynomial
  //
  //                 n0     (alpha,beta)           n1
  //   p  (x)  =  (x)   *  p (x)         *  (1 - x)
  //    nt                   n
  //
  // at the interpolation points.
  //
  // See Villadsen and Michelsen, pages 131-132 and 418.
  //
  // Input parameters:
  //
  //   nd     : the dimension of the vectors dif1, dif2, dif3, and root
  //
  //   n      : the degree of the jacobi polynomial, (i.e., the number
  //            of interior interpolation points)
  //
  //   n0     : determines whether x = 0 is included as an
  //            interpolation point
  //
  //              n0 = 0  ==>  x = 0 is not included
  //              n0 = 1  ==>  x = 0 is included
  //
  //   n1     : determines whether x = 1 is included as an
  //            interpolation point
  //
  //              n1 = 0  ==>  x = 1 is not included
  //              n1 = 1  ==>  x = 1 is included
  //
  //   alpha  : the value of alpha in the description of the jacobi
  //            polynomial
  //
  //   beta   : the value of beta in the description of the jacobi
  //            polynomial
  //
  //   For a more complete explanation of alpha an beta, see Villadsen
  //   and Michelsen, pages 57 to 59.
  //
  // Output parameters:
  //
  //   root   : one dimensional vector containing on exit the
  //            n + n0 + n1 zeros of the node polynomial used in the
  //            interpolation routine
  //
  //   dif1   : one dimensional vector containing the first derivative
  //            of the node polynomial at the zeros
  //
  //   dif2   : one dimensional vector containing the second derivative
  //            of the node polynomial at the zeros
  //
  //   dif3   : one dimensional vector containing the third derivative
  //            of the node polynomial at the zeros

  static bool jcobi (octave_idx_type n, octave_idx_type n0, octave_idx_type n1,
                     double alpha, double beta, double *dif1, double *dif2,
                     double *dif3, double *root)
  {
    assert (n0 == 0 || n0 == 1);
    assert (n1 == 0 || n1 == 1);

    octave_idx_type nt = n + n0 + n1;

    assert (nt >= 1);

    // -- first evaluation of coefficients in recursion formulas.
    // -- recursion coefficients are stored in dif1 and dif2.

    double ab = alpha + beta;
    double ad = beta - alpha;
    double ap = beta * alpha;

    dif1[0] = (ad / (ab + 2.0) + 1.0) / 2.0;
    dif2[0] = 0.0;

    if (n >= 2)
      {
        for (octave_idx_type i = 1; i < n; i++)
          {
            double z1 = i;
            double z = ab + 2 * z1;

            dif1[i] = (ab * ad / z / (z + 2.0) + 1.0) / 2.0;

            if (i == 1)
              dif2[i] = (ab + ap + z1) / z / z / (z + 1.0);
            else
              {
                z *= z;
                double y = z1 * (ab + z1);
                y *= (ap + y);
                dif2[i] = y / z / (z - 1.0);
              }
          }
      }

    // Root determination by Newton method with suppression of previously
    // determined roots.

    double x = 0.0;

    for (octave_idx_type i = 0; i < n; i++)
      {
        bool done = false;

        int k = 0;

        while (! done)
          {
            double xd = 0.0;
            double xn = 1.0;
            double xd1 = 0.0;
            double xn1 = 0.0;

            for (octave_idx_type j = 0; j < n; j++)
              {
                double xp  = (dif1[j] - x) * xn  - dif2[j] * xd;
                double xp1 = (dif1[j] - x) * xn1 - dif2[j] * xd1 - xn;

                xd  = xn;
                xd1 = xn1;
                xn  = xp;
                xn1 = xp1;
              }

            double zc = 1.0;
            double z = xn / xn1;

            if (i != 0)
              {
                for (octave_idx_type j = 1; j <= i; j++)
                  zc -= z / (x - root[j-1]);
              }

            z /= zc;
            x -= z;

            // Famous last words:  100 iterations should be more than
            // enough in all cases.

            if (++k > 100 || math::isnan (z))
              return false;

            if (std::abs (z) <= 100 * std::numeric_limits<double>::epsilon ())
              done = true;
          }

        root[i] = x;
        x += std::sqrt (std::numeric_limits<double>::epsilon ());
      }

    // Add interpolation points at x = 0 and/or x = 1.

    if (n0 != 0)
      {
        for (octave_idx_type i = n; i > 0; i--)
          root[i] = root[i-1];

        root[0] = 0.0;
      }

    if (n1 != 0)
      root[nt-1] = 1.0;

    dif (nt, root, dif1, dif2, dif3);

    return true;
  }

  // Compute derivative weights for orthogonal collocation.
  //
  // See Villadsen and Michelsen, pages 133-134, 419.
  //
  // Input parameters:
  //
  //   nd     : the dimension of the vectors dif1, dif2, dif3, and root
  //
  //   n      : the degree of the jacobi polynomial, (i.e., the number
  //            of interior interpolation points)
  //
  //   n0     : determines whether x = 0 is included as an
  //            interpolation point
  //
  //              n0 = 0  ==>  x = 0 is not included
  //              n0 = 1  ==>  x = 0 is included
  //
  //   n1     : determines whether x = 1 is included as an
  //            interpolation point
  //
  //              n1 = 0  ==>  x = 1 is not included
  //              n1 = 1  ==>  x = 1 is included
  //
  //   i      : the index of the node for which the weights are to be
  //            calculated
  //
  //   id     : indicator
  //
  //              id = 1  ==>  first derivative weights are computed
  //              id = 2  ==>  second derivative weights are computed
  //              id = 3  ==>  gaussian weights are computed (in this
  //                           case, the value of i is irrelevant)
  //
  // Output parameters:
  //
  //   dif1   : one dimensional vector containing the first derivative
  //            of the node polynomial at the zeros
  //
  //   dif2   : one dimensional vector containing the second derivative
  //            of the node polynomial at the zeros
  //
  //   dif3   : one dimensional vector containing the third derivative
  //            of the node polynomial at the zeros
  //
  //   vect   : one dimensional vector of computed weights

  static void dfopr (octave_idx_type n, octave_idx_type n0, octave_idx_type n1,
                     octave_idx_type i, octave_idx_type id, double *dif1,
                     double *dif2, double *dif3, double *root, double *vect)
  {
    assert (n0 == 0 || n0 == 1);
    assert (n1 == 0 || n1 == 1);

    octave_idx_type nt = n + n0 + n1;

    assert (nt >= 1);

    assert (id == 1 || id == 2 || id == 3);

    if (id != 3)
      assert (i >= 0 && i < nt);

    // Evaluate discretization matrices and Gaussian quadrature weights.
    // Quadrature weights are normalized to sum to one.

    if (id != 3)
      {
        for (octave_idx_type j = 0; j < nt; j++)
          {
            if (j == i)
              {
                if (id == 1)
                  vect[i] = dif2[i] / dif1[i] / 2.0;
                else
                  vect[i] = dif3[i] / dif1[i] / 3.0;
              }
            else
              {
                double y = root[i] - root[j];

                vect[j] = dif1[i] / dif1[j] / y;

                if (id == 2)
                  vect[j] = vect[j] * (dif2[i] / dif1[i] - 2.0 / y);
              }
          }
      }
    else
      {
        double y = 0.0;

        for (octave_idx_type j = 0; j < nt; j++)
          {
            double x = root[j];

            double ax = x * (1.0 - x);

            if (n0 == 0)
              ax = ax / x / x;

            if (n1 == 0)
              ax = ax / (1.0 - x) / (1.0 - x);

            vect[j] = ax / (dif1[j] * dif1[j]);

            y += vect[j];
          }

        for (octave_idx_type j = 0; j < nt; j++)
          vect[j] = vect[j] / y;
      }
  }

  // Error handling.

  void CollocWt::error (const char *msg)
  {
    (*current_liboctave_error_handler) ("CollocWt: fatal error '%s'", msg);
  }

  CollocWt& CollocWt::set_left (double val)
  {
    if (val >= m_rb)
      error ("CollocWt: left bound greater than right bound");

    m_lb = val;
    m_initialized = 0;
    return *this;
  }

  CollocWt& CollocWt::set_right (double val)
  {
    if (val <= m_lb)
      error ("CollocWt: right bound less than left bound");

    m_rb = val;
    m_initialized = 0;
    return *this;
  }

  void CollocWt::init (void)
  {
    // Check for possible errors.

    double wid = m_rb - m_lb;
    if (wid <= 0.0)
      {
        error ("CollocWt: width less than or equal to zero");
      }

    octave_idx_type nt = m_n + m_inc_left + m_inc_right;

    if (nt < 0)
      error ("CollocWt: total number of collocation points less than zero");
    else if (nt == 0)
      return;

    Array<double> dif1 (dim_vector (nt, 1));
    double *pdif1 = dif1.fortran_vec ();

    Array<double> dif2 (dim_vector (nt, 1));
    double *pdif2 = dif2.fortran_vec ();

    Array<double> dif3 (dim_vector (nt, 1));
    double *pdif3 = dif3.fortran_vec ();

    Array<double> vect (dim_vector (nt, 1));
    double *pvect = vect.fortran_vec ();

    m_r.resize (nt, 1);
    m_q.resize (nt, 1);
    m_A.resize (nt, nt);
    m_B.resize (nt, nt);

    double *pr = m_r.fortran_vec ();

    // Compute roots.

    if (! jcobi (m_n, m_inc_left, m_inc_right, m_alpha, m_beta, pdif1,
                 pdif2, pdif3, pr))
      error ("jcobi: newton iteration failed");

    octave_idx_type id;

    // First derivative weights.

    id = 1;
    for (octave_idx_type i = 0; i < nt; i++)
      {
        dfopr (m_n, m_inc_left, m_inc_right, i, id, pdif1, pdif2, pdif3,
               pr, pvect);

        for (octave_idx_type j = 0; j < nt; j++)
          m_A(i,j) = vect(j);
      }

    // Second derivative weights.

    id = 2;
    for (octave_idx_type i = 0; i < nt; i++)
      {
        dfopr (m_n, m_inc_left, m_inc_right, i, id, pdif1, pdif2, pdif3,
               pr, pvect);

        for (octave_idx_type j = 0; j < nt; j++)
          m_B(i,j) = vect(j);
      }

    // Gaussian quadrature weights.

    id = 3;
    double *pq = m_q.fortran_vec ();
    dfopr (m_n, m_inc_left, m_inc_right, id, id, pdif1, pdif2, pdif3, pr, pq);

    m_initialized = 1;
  }

  std::ostream& operator << (std::ostream& os, const CollocWt& a)
  {
    if (a.left_included ())
      os << "left  boundary is included\n";
    else
      os << "left  boundary is not included\n";

    if (a.right_included ())
      os << "right boundary is included\n";
    else
      os << "right boundary is not included\n";

    os << "\n";

    os << a.m_alpha << ' ' << a.m_beta << "\n\n"
       << a.m_r << "\n\n"
       << a.m_q << "\n\n"
       << a.m_A << "\n"
       << a.m_B << "\n";

    return os;
  }
}
